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Stormwater Runoff: The Definitive Guide 2023

Stormwater runoff is a term synonymous with the movement of water, primarily occurring when rainwater or snowmelt traverses across land and man-made surfaces, instead of seeping into the earth.

What is Stormwater Runoff?

Stormwater runoff is a term synonymous with the movement of water, primarily occurring when rainwater or snowmelt traverses across land and surfaces, instead of seeping into the earth.

The Journey of Runoff

A significant share of this traveling water is ushered by impervious or non-absorbent surfaces.

Impervious Surfaces

These include concrete, asphalt, rooftops, and compacted soil that vehemently deter the water from infiltrating the ground.

The Urbanization Factor

Urban sprawl has exacerbated the issue, where the burgeoning infrastructure like roads, parking lots, buildings, and sidewalks, interrupts the natural water cycle drastically.

A Disrupted Water Cycle

In contrast, natural environments like forests and grasslands have a more harmonious relationship with rainwater.

Here, the water is either absorbed by the vegetation, evaporates, or gently percolates into the ground replenishing groundwater reserves.

The Swiftness of Urban Runoff

However, in urban settings, the story is starkly different.

The water hastily accumulates on impervious surfaces, swiftly moving into storm drains or nearby water bodies, devoid of any filtration.

Pollutants in Stormwater Runoff

In addition to increasing runoff volumes, impervious surfaces accumulate pollutants between rain events that are picked up by runoff.

Studies by the U.S. Environmental Protection Agency have detected high levels of sediment, nutrients, bacteria, metals, hydrocarbons, pesticides, and other toxic chemicals in urban stormwater runoff. Common pollutant sources include:

  • Sediment from construction sites, winter road sanding, and stream bank erosion
  • Nutrients like nitrogen and phosphorus from lawn fertilizers
  • Bacteria from pet waste, faulty septic systems, and leaking sanitary sewers
  • Metals such as zinc, copper, lead, cadmium from vehicle brake pads, roof shingles, and industrial activities
  • Hydrocarbons like oil and grease from leaking vehicles and machinery
  • Pesticides from lawns, roadsides, and agricultural areas
  • Toxic chemicals from spills, leaks, improper disposal, and industrial activities


Impacts of Stormwater Pollution

When discharged untreated into lakes, rivers, and coastal waters, polluted stormwater runoff degrades water quality and harms plants, fish, wildlife, and humans.

Major impacts include:

  • Algae blooms fed by excess nutrients like nitrogen and phosphorus
  • Bacteria contamination that makes waters unsafe for swimming and fishing
  • Turbidity and sedimentation that reduce light penetration and smother aquatic habitat
  • Toxic chemicals that accumulate in fish tissue and poison aquatic life
  • Trash and debris that choke waterways and harm wildlife
  • Eroded stream banks and streambed scouring caused by high flow volumes
  • Contaminated drinking water supplies requiring expensive treatment
  • Flooding damage to properties and infrastructure

The Regulatory Landscape

The negative effects of stormwater runoff have led to regulations at federal, state, and local levels.

The Clean Water Act established the National Pollutant Discharge Elimination System (NPDES) to regulate discharges from municipal separate storm sewer systems (MS4s), construction activities, and industrial facilities.

MS4 permits require municipalities to develop stormwater management plans that include:

  1. public education
  2. construction site runoff controls
  3. post-construction runoff management
  4. pollution prevention measures

Many states and local authorities also have additional requirements to reduce stormwater pollution from development projects.

These programs establish design standards for runoff reduction, typically through low impact development (LID) and green infrastructure techniques.

Developers are required to implement best management practices (BMPs) to manage stormwater onsite and limit runoff volumes.

Best Management Practices for Stormwater

BMPs encompass a variety of techniques to reduce and filter stormwater runoff.

They can be structural systems that are designed and engineered, as well as non-structural practices like landscape management.

Commonly used BMPs include:

  • Bioswales – Vegetated drainage ditches to slow, filter, and infiltrate runoff
  • Retention and detention ponds – Store and treat stormwater
  • Constructed wetlands – Use natural processes to filter runoff
  • Permeable pavements – Allow infiltration through porous surfaces
  • Rain gardens and bioretention – Shallow planted depressions that absorb runoff
  • Green roofs – Rooftop gardens to absorb rainfall and reduce runoff
  • Riparian buffers – Streamside vegetation to filter runoff and prevent erosion
  • Downspout disconnection – Redirect rooftop runoff to permeable areas
  • Native landscaping – Deep rooted vegetation with less need for fertilizers/pesticides
  • Street sweeping – Removes debris, sediments, and metals on road surfaces
  • Public education – Encourages residential BMPs like rain barrels and picking up pet waste

Why is Stormwater Runoff a Problem?

Environmental Impact

  • Pollution: Stormwater runoff picks up pollutants like oil, grease, chemicals, and metals from roadways and other surfaces.
  • Erosion and Sedimentation: The fast-moving water can erode soil and carry sediment into waterways.
  • Habitat Destruction: High volumes of runoff can lead to flooding, destroying natural habitats for plants and animals.

Human Impact

  • Water Quality: Polluted runoff can contaminate drinking water supplies.
  • Flooding: Inadequate stormwater systems can lead to flooding, causing property damage and even loss of life.

How Can We Manage It?

Structural Solutions

  1. Retention Ponds: These are designed to hold water temporarily to slow down stormwater runoff and improve the quality of the water through sedimentation.
  2. Green Roofs: These roofs are covered with vegetation that absorbs rainwater, reducing runoff.

Non-Structural Solutions

  1. Public Awareness: Educating the public about the dangers of stormwater runoff is crucial.
  2. Good Housekeeping: Simple actions like not dumping oil down drains can make a difference.

Technological Advancements

New technologies are being developed to treat and manage stormwater.

Smart drainage systems use sensors and real-time monitoring to control water flow, reducing flooding and pollution.

It’s not just a problem for governments and organizations to solve; it requires collective action from each one of us.

With proper management and awareness, we can mitigate the negative impacts of stormwater runoff and protect our precious water resources for future generations.

More information

Stormwater Management Program

Construction site stormwater runoff control

Construction best management practices (BMPs) for environmental resources

Source: Texas Department of Transportation (TxDOT)

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